Modern wireless communications standards and systems rely on intricate scheduling schemes to efficiently utilize the limited radio resources and maximize the system throughput. 4G cellular standards feature orthogonal frequency division multiple access (OFDMA) where resource scheduling is needed for both downlink and uplink transmission. The scheduling is done at the base station in a centralized fashion, and the scheduling grants can be for downlink transmission or uplink transmission. Both downlink and uplink grants are transmitted in Physical Downlink Control CHannel (PDCCH). The adjective “physical” before “downlink control channel” emphasizes that such control signaling occurs very frequently, i.e., every 1 ms, so that it can dynamically adapt to the fast fading of the channel. Frequent signaling, although crucial for dynamic link adaptation, is very expensive, in the sense that its overhead is high, in particular for those mobile terminals at cell edges. Hence, there should be a limit for the overhead of physical downlink control channel, so that there are enough physical resources for downlink data transmissions.
In 3GPP LTE, physical downlink control channel (PDCCH) is time multiplexed with Physical Downlink Shared Channel (PDSCH) which carries the downlink data. More specifically, the physical control channel occupies the entire first few OFDM symbols, spanning over the whole system bandwidth. Downlink and uplink grants of different users are encoded, modulated, cross-interleaved and mapped to those OFDM symbols, designated for PDCCH. The design principles of PDCCH are: 1) to concentrate PDCCH in the first few OFDM symbols to allow earlier decoding of DL/UL grants; 2) to ensure robust decoding of PDCCH. The purpose of the cross-interleaving and the spreading over entire system bandwidth is to randomize the frequency-selective fading and interference, and to achieve robustness.
In LTE-A relay, a new physical control channel is introduced to the relay backhaul link, called relay physical downlink control channel (R-PDCCH) [1]. R-PDCCH takes some resources in the downlink data channel (PDSCH) region. R-PDCCH can occupy an entire physical resource block (PRB) pair consisting of two slots, or just one slot of PRB. When both slots are occupied by R-PDCCH, the multiplexing between R-PDCCH and PDSCH is called frequency division multiplexing (FDM). When only one slot (especially the first slot) is occupied by R-PDCCH, the multiplexing between R-PDCCH and PDSCH is called time division multiplexing (TDM)+frequency division multiplexing (FDM). To reduce the decoding latency, DL grants for relay backhaul link can only be transmitted in the first slot(s) of PRB pair(s), whereas UL grants for relay backhaul can only be transmitted in the second slot(s) of PRB pair(s). R-PDCCH can be cross-interleaved or non cross-interleaved over different relay nodes. For cross-interleaved R-PDCCH, only common reference signal (CRS) can be used for demodulation. The cross-interleaved R-PDCCH bears a lot resemblance to PDCCH where the decoding robustness is a top consideration. Due to CRS demodulation, cross-interleaved R-PDCCH cannot benefit from beamforming, similar to PDCCH. Cross-interleaving also prevents the frequency selective scheduling gain. In contrast, the design principle of non cross-interleaved R-PDCCH is to exploit beamforming and frequency selective gain where demodulation reference signal (DMRS) is preferred. DMRS is present only in physical resource blocks that contain R-PDCCH or PDSCH, and goes through the same precoding as R-PDCCH or PDSCH. Therefore, precoding is totally transparent to R-PDCCH, i.e., no need for separate signaling to indicate precoding matrices used at the transmitter.
Traditional cellular networks consist primarily of macro base stations whose transmit power and antenna gains are the same. The site-to-site distance is almost constant and the network grids are quite uniform. The fast growing demand for system capacity motivates the deployment of a large number of low power nodes such as remote radio head (RRH) pico, femto nodes, to offload the traffic from the macro and increase the throughput in hot-spots. The mixture of macro, remote radio head, pico and femto cells constitutes the so called heterogeneous networks (HetNets). Interference scenarios become more complex in HetNets, not only for data channels such as PDSCH, but also for control channels such as PDCCH. There are several interference coordination schemes for PDSCH in HetNets deployment, one of them being partial frequency reuse where orthogonal resources are allocated for users near cell edges. However, such scheme cannot be used for PDCCH since it occupies the entire system bandwidth. In another HetNet scenario, remote radio head (RRH) is with the same cell ID as of macro cell, thus appearing transparent to users. Transparent RRH can significantly improve the system throughput by dynamic joint transmission over macro antennas and RRH. But, it has issue with limited resources for PDCCH which is shared by all scheduled users within the coverage area, i.e., there is no cell splitting gain of PDCCH. So there is a strong motivation of improving the capacity of PDCCH and mitigating the strong interference in HetNets. The enhanced physical downlink control channel is called ePDCCH.